The present invention relates to a charged particle beam apparatus for controllably scanning a charged particle beam (such as an electron beam or ion beam) over a specimen to obtain a final image and, more particularly, to a charged particle beam apparatus adapted to obtain stereoscopic, three-dimensional images.
In a charged particle beam apparatus typified by a scanning electron microscope, a sharply focused charged particle beam is scanned over a specimen to obtain desired information (e.g., images of the specimen) from the specimen. In such a charged particle beam apparatus, stereoscopic imaging has been heretofore done utilizing a crossed view method, a parallel view method, or an anaglyph method using the human eye visual perception. In the crossed method or parallel method, the specimen is tilted left and right with respect to the scanning beam. Two still images or two still photographs are acquired from different orientations. Then, stereo viewing is achieved by making use of the parallax between the left and right eyes. In the anaglyph method, red-blue glasses are used.
In the prior art, in a case where a two-dimensional image is obtained by scanning a charged particle beam, it is customary to control the beam for the scanning such that the optical axis of the beam passes through the axis of the objective lens. In recent years, a novel scanning technique has been proposed. In particular, the charged particle beam is tilted about a given position on the axis of the optical system by a deflection coil. That is, a tilt angle is given to the beam to achieve beam rocking. The tilted beam is off-axis incident on the objective lens. The objective lens rocks the beam back to its original posture. In this way, the charged particle beam is scanned over the specimen. Techniques regarding correction concomitant with the off-axis aberration and optical axis adjustment are also proposed.
In the field of general image display devices, techniques that can be applied for stereoscopic imaging have evolved in their respective technical fields because of appearance of three-dimensional liquid crystal displays. It is easy to forecast that a shift will be made from the era of stereoscopic imaging relying on the prior-art capturing of photographs to the era of real-time stereoscopic imaging by combining those techniques. Especially, in scanning electron microscopy, real-time stereoscopic imaging will find wide application. In the prior art, flat two-dimensional images (so-called general scanning electron microscope images) have been acquired. Accordingly, in the fields of bioscience and metal materials, there is a strong demand for a technique permitting evaluation of images having information in the thickness and depth directions while making an observation. The present invention is directed towards this technique.
Typical examples of the prior art are disclosed in JP-U-55-48610 and JP-A-2-33843. A technique consisting of irradiating a specimen with a tilted charged particle beam and employing the action of the objective lens rocking the beam back to its original posture at off-axis positions is known. An optical axis adjusting means appropriate for this technique is disclosed in JP-A-2006-12664. In particular, a combination of plural lenses and cancellation means are derived. The cancellation means analytically resolves the behavior of a charged particle beam off-axis incident on an objective lens and thus permits optical components to provide overall cancellation of aberrations produced when the beam is tilted over the specimen. This is well known as an imaging method utilizing tilt of beam on one side of the axis and has been already put into practical use.